Semiconductor devices formed on a semiconductor-on-insulator (SOI) substrate provide advantages in performance since a buried insulator layer separating a handle substrate and a top semiconductor layer provides reduced capacitive coupling between devices formed on the top semiconductor layer and the handle substrate. For example, the dielectric constant of silicon is about 11.68, while the dielectric constant of silicon oxide is about 3.9. Since capacitive coupling between a semiconductor device and the semiconductor substrate containing the semiconductor device is proportional to the dielectric constant, or the relative permittivity, of the material comprising the semiconductor substrate, a semiconductor device formed on an SOI substrate may provide performance advantage, and alternatively or concurrently, lower power consumption.
However, SOI field effect transistors in general suffer from floating body effects. The body of an SOI field effect transistor stores electrical charge as a function of the history of the device, hence changing the body voltage accordingly and becoming a “floating” body. As such, an SOI field effect transistor has threshold voltages which are difficult to anticipate and control, and which vary in time. The body charge storage effects result in dynamic sub-threshold voltage (sub-Vt) leakage and threshold voltage (Vt) mismatch among geometrically identical adjacent devices. Floating body effects in an SOI field effect transistor are particularly a concern in static random access memory (SRAM) cells, where Vt matching is extremely important as operating voltages continue to be scaled down. Another concern with the SOI field effect transistor is with stacked devices, as used in logic gates, in which the conductive state of devices higher up in the stack are strongly influenced by stored body charge, because of reduced gate-to-source voltage (Vgs) overdrive available to these devices.
While it may be desirable to employ conventional SOI field effect transistors for some devices in a semiconductor circuit, a field effect transistor with pre-determined body voltage is also desired on the same SOI substrate for some other devices. For example, it is desired to have a controllable body voltage in a floating body memory device.
In view of the above, there exists a need for a field effect transistor having a controllable body voltage that may be formed on an SOI substrate. Further, there exists a need for such an SOI field effect transistor having a controllable body voltage formed on the same SOI substrate containing a conventional SOI field effect transistor having typical floating body effects. Yet further, there exist a need for manufacturing such an SOI field effect transistor having a controllable body voltage—either alone or in conjunction with a conventional SOI field effect transistor having typical floating body effects.